It’s All About The Base

From the editor – Trainers see many different machinery base designs and installations during the OJT portion of our training classes. Some bases are quite substantial while others are flimsy and actually downright horrible Poor bases have a negative impact on rotating machinery reliability and can certainly be a cause of precision shaft alignment frustration.

Two of our trainers (Mac and James) recently had experiences with aligning rotating machinery with bad bases, leading both to submit blogs on the issue. Part 1 is from Mac who did some research on proper base design and found an excellent article on base installation at www.waterworld.com titled “Pump Baseplates and Installation Can Impact Pump Reliability” by Allan R. Budris, P.E.

It follows here for your information:

Pump Baseplates and Installation Can impact Pump Reliability

According to statistical reliability analysis, as much as 65% of pump life cycle costs are determined during the design, procurement, and installation phases of new machinery applications. While design and procurement are important aspects for any application, the installation of the equipment plays a very significant role. A superb design, poorly installed, will give poor results. A moderate design, properly installed, will give good results.

The long-term success of a proper installation and reduced vibration activity is determined by how well the machinery system is joined to the foundation. The baseplate, or skid, of the machinery system must become a monolithic member of the foundation system. Machinery vibration should ideally be transmitted through the baseplate to the foundation and down through the subsoil. Failure to do so will result in the machinery resonating on the baseplate, very often causing consequential damage.

The issue always boils down to cost: long term, reliability influenced, “Life Cycle Costs” versus “First Costs”. Do you select the least expensive (weakest) or high quality baseplate; spend the time required to properly prepare the foundation and baseplate for the grout; select superior epoxy grout versus low cost cement grout; or use a lower labor cost single pour, instead of the more effective two pour method.

Pump Baseplate Types

Sound baseplate selection is one of the first steps in a good pump installation. Pump baseplates are available in a variety of materials and configurations, such as cast iron, fabricated steel, molded polymer concrete, “PIP-Compliant Fabricated Steel,” Pre-grouted, and free-standing / stilt-mounted configurations. Torsional stiffness, rigidity and flatness are important for all of these and grouting and lifting issues must be addressed for most of them. Every one of these configurations has its advantages and disadvantages:

Cast iron probably provides the best vibration dampening, although can warp during the machining process. It should, therefore, be properly leveled, grouted, and finish-machined in place.

Fabricated steel must be sufficiently heavy, properly welded, stress relieved and machined. Flimsy fabricated steel bases have led to the development of “enhanced” versions.

PIP-compliant fabricated steel baseplates provide a high level of enhancements, and features.

Free-standing, or stilt-mounted baseplates are available from pump manufacturers that wish to respond to user demands for low-cost installations. Stilt-mounted bases can involve a number of options, and the entire assembly is expected to move on the floor. The movement “tends” to equalize the forces exerted by cheaply installed piping, and is expected to provide equal loads on both suction and discharge flanges. This assumes that all pump piping loads act in the same direction, which is seldom the case. While stilt-mounted bases may not have to contend with grout problems, they are rarely, if ever, the best choice for reliability focused users. Only the smallest pumps should be placed on free-standing baseplates.

Pre-grouted pump baseplates have proven to be a highly effective method to minimize field installation costs and improve reliability. They can provide a pump train that is 60% mechanically complete when it arrives at the plant site. Such baseplates typically include a detailed inspection of the primer system used on the underside of the baseplate, proper preparation of the primer for grouting, grouting of the baseplate, post-curing of the grout material, and a detailed post-grout inspection of the baseplate mounting surfaces, with finish machining performed if the mounting surface tolerances fall outside of the flatness specifications.

The process of shipping, lifting, storing, and setting the baseplate can have a negative impact on the motor mounting surfaces. While these surfaces may have initially been flat, experience shows that there is often work to be done by the time the baseplate reaches the field, unless the baseplate is pre-grouted.

Foundation

The Total Foundation is the complete unit assembly which supports a machine and is made up of a machine base, adapter section, chock plate, principal foundation, and sub-foundation. Great design effort and cost is typically expended in the construction of a machinery foundation. The important factors to be consider when designing a foundation are: the foundation should be adequately designed to support the machinery; the foundation should ideally rest on bedrock or solid earth to avoid resonant vibration; and the driver, gearbox and pump should rest on one common foundation. The foundation mass should be a minimum of three times that of the pump assembly (pump plus motor).

Irrespective of baseplate style, correct preparation of the top of the concrete foundation will have long-term reliability implications and is important. The laitance of this surface must be removed by chipping for proper bonding.

Grout Material and Preparation

Now let us turn to the grout material and preparation. There are two basic types of grout, “Portland Cement” and “Epoxy Grout”, with epoxy grout being the superior, but more expensive. Cement grouts will degrade when allowed to come into contact with lubricating oils and many pumped products. The underside of the baseplate must be cleaned, and the surface must be free of oils, grease, moisture, and other contaminants. All of these contaminants greatly reduce the tensile bond strength of the grout system, which can lead to voids and bonding issues. All exposed grout and concrete surfaces shall be sealed with one coat of a premium unfilled Epoxy primer, having sufficient tensile bond strength. The issue always boils down to cost: Life Cycle Costs versus first cost.

Grout Pour Methods

The two-pour method is the most widely used, and can utilize either a cementitious or epoxy grout. The wooden grout forms for the two-pour method are easier to build because of the open top. Forms shall be made liquid tight to prevent leaking of grout material. Cracks and openings shall be sealed off with rags, cotton batting, foam rubber or caulking compound.

The one-pour grouting method requires a more elaborate form building technique, but does reduce labor cost. The wooden grout form now requires a top plate that forms a liquid tight seal against the bottom flange of the base plate. This pour technique requires good flow characteristics from the grout material, and is typically only used for epoxy grout applications. During the course of a conventional grouting procedure, it is very common to exceed the inches of head necessary to lift a pre-filled base plate. For this reason, it is very important to ascertain that the base plate is locked down.

It is very important to avoid voids or air pockets in the grout. Voids inhibit the foundation system from damping resonance and shaft-generated vibration. Insufficient vent holes or static head are execution issues that can be addressed, through proper installation techniques, to avoid large voids. The most overlooked causes of voids are related to bonding issues. Another field installation problem with costly implications is distortion of the base plate machine surfaces. Distortion can either be induced prior to grouting due to poor field leveling techniques, or the distortion can be generated by the grout itself. All epoxy grout systems have a slight shrinkage factor. Baseplates with sturdy cross braces are not affected by the slight volume change of the grout. For less rigid designs, the bond strength of the epoxy grout can be stronger than the base plate itself.

Conclusions

The advantages of a high quality, void-free, fully co-planar, pump baseplate installation are: lower energy losses to friction and vibration; increased productivity through time savings from repair avoidance; and reduced parts expense and lower inventory requirements.